1. Field of the Invention
The present invention relates to an FM detector circuit using a discriminator.
2. Description of the Related Art
Known FM detector circuits use a discriminator as a phase shifter for detecting a deviation in frequency of an FM signal as a variation in voltage. A piezoelectric material of the discriminator typically has a low Q and a wide band Δf (=Fa (antiresonance frequency)−Fr (resonance frequency)) that results in a wide demodulated output bandwidth. Various types of temperature characteristics of each piezoelectric material are not appropriate, and the temperature characteristic (foTC) of the discriminator is relatively large. The range of temperature within which an FM apparatus remains operable is narrower than that of a ceramic filter for FM application. The FM apparatuses are thus subject to a very narrow operation temperature range limit.
Conventionally, the temperature characteristic (foTC) of the discriminator is about 25 ppm/° C. or so. A discriminator having an fo of 10.7 MHz is subject to a frequency variation of about 28 kHz within a temperature range of about 100° C., and a frequency variation of about 40 kHz within a temperature range of about 150° C. Conventional discriminators tend to suffer from more frequency variation in a high temperature range above 20° C. To satisfy the typically applied standard of foTC, namely, variations of about ±30 kHz of the center frequency fo, the upper operating temperature is limited to about 60° C.
The inventors of the present invention have proposed a method that stabilizes frequency-temperature characteristics of a discriminator (a piezoelectric resonator), as described in Japanese Patent Application No. 2001-89064 which is not published yet.
The disclosed method is based on the finding that a difference between a temperature characteristic foTC of the center frequency of the discriminator and a mean value of a temperature characteristic FrTC of a resonance frequency and a temperature characteristic FaTC of an antiresonance frequency is proportional to a product of a temperature characteristic εTC of a capacitance of the discriminator and a specific frequency bandwidth Δf/fo of the discriminator. The temperature characteristic foTC of the center frequency is approximated from the temperature characteristic FrTC of the resonance frequency, the temperature characteristic FaTC of the antiresonance frequency, the temperature characteristic εTC of the capacitance and the specific frequency bandwidth Δf/fo. The temperature characteristic of foTC is set to be within a target value.
Referring to FIG. 1A, a known FM detector circuit of a balanced bridge type includes resistors R1, R2, and R3 respectively connected as three sections, and a discriminator D connected as one remaining section. FIG. 1A is a circuit diagram, and FIG. 1B illustrates a change in the phase of an output voltage Eo. As shown, the FM detector is designed so that the output voltage Eo at the center frequency fo, which lags an input voltage Ei by 90°, is picked up.
An impedance value at the center frequency fo is determined by resistance values of the resistors R1, R2, and R3 included in a detector IC. The resistors R1, R2, and R3 are typically in the vicinity of 1 kΩ.
Some detector ICs have temperature characteristic unique to the internal circuit thereof, and the internal resistors determining the center frequency fo vary in resistance. The variations are attributed to the temperature characteristics of the resistors R1, R2, and R3 defining the bridge circuit. In some detector ICs, a capacitor is connected in parallel with each of the resistors R1, R2, and R3, or a capacitor is substituted for one of the resistors R1, R2, and R3. In this arrangement, the temperature characteristic of the capacitor greatly affects the detector IC.
If the detector IC itself has the temperature characteristic that varies the resistance determining the center frequency fo, the temperature characteristic foTC of the entire detector circuit including the detector IC is adversely affected even though the temperature characteristic foTC of the discriminator is stabilized.